SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

8.7. DESIGN ISSUES IN HFETS 397

(a)

(b)

n+ GaAs cap

AlGaAs

GaAs

2DEG

δ-layer

A

A'

Rint Rint

Rch Rch Rch

Rn+ Rn+

EF(A)

EF(A')

AA'

SD

G

Rint

Figure 8.24: (a) Schematic of an AlGaAs/GaAs HFET with ann+cap layer and a recessed gate.
Also shown are the various resistive components that make up the source access resistance. (b)
Band diagram across the structure.

The 2DEG conductivity is at a maximum when theμn·nsproduct is maximized. From our
discussion of the Lever Rule in section 8.5, it is clear that the 2DEG densitynsincreases as the
δ-doping sheet is brought closer to the heterointerface. However, decreasing the spacer distance
dsalso causes the electron mobilityμnto decrease because of the increase in remote ionized
impurity scattering. It is therefore clear that the 2DEG conductivity will have a maximum at a
valueds(optimum) which must be determined for each material system and doping level. Typical
values are 5 nm for the AlInAs/GaInAs system, 3 nm for the AlGaAs/InGaAs system, and 2 nm
for the AlGaAs/GaAs system.